The effect of 3'-terminal monophosphate group on the metal-ion-promoted hydrolysis of the phosphodiester bonds of oligonucleotides has been studied. For this purpose, the rate constants for the hydrolysis of the following oligomers in the presence of Zn2+ and its 1,5,9-triazacyclododecane chelate, Zn2+[12]aneN(3), have been determined: (i) ApUpUp(2') and ApUpUp(3'), (ii) Up(Tp)(n)T (n = 0-4 and 7), (iii) Up(Tp)(n)Tp (n = 0-4 and 7). The results obtained are used to propose a mechanism for the Zn2+ and Zn2+ [12]aneN(3) promoted hydrolysis of polynucleotides.